1. Technical Field
The present invention relates to a method and a monitoring arrangement for monitoring the functioning of a sensor arranged to measure the magnitude of a parameter related to exhaust gases flowing out of an exhaust gas aftertreatment appliance included in an exhaust gas aftertreatment system of a motor vehicle. The invention also relates to a method and a monitoring arrangement for monitoring the functioning of an exhaust gas aftertreatment system of a motor vehicle based on measuring values from a sensor as to the magnitude of a parameter related to exhaust gases flowing out of an exhaust gas aftertreatment appliance included in the exhaust gas aftertreatment system. Furthermore, the invention relates to a computer program comprising computer program code for implementing a method according to the invention, a computer program product comprising a data storage medium readable by an electronic control unit and having said computer program stored thereon, and an electronic control unit.
2. Background Art
In order to fulfil existing and future regulations regarding On-Board Diagnostics of an exhaust gas aftertreatment system of a heavy motor vehicle, it is for instance necessary to control that a catalyst, e.g. in the form of an SCR catalyst (SCR=Selective Catalytic Reduction), included in the system is functioning properly.
A method for monitoring an SCR catalyst is previously known from WO 2004/109072 A1. According to this known method, a temperature value representing the temperature of exhaust gases flowing out of the SCR catalyst is calculated by means of a calculation model and compared with a temperature value measured in the exhaust line downstream of the SCR catalyst. Based on the correspondence between these temperature values, such as for instance the difference between them, it is established whether or not the SCR catalyst and its injection device are functioning in a satisfying and expected manner. Similar methods are also previously known from DE 4 122 787 A1, U.S. Pat. No. 5,860,277 A and EP 0 756 071 A2. A problem associated with this type of monitoring methods relying on a calculation model is that the calculation model normally might give rather correct calculation values but at some operating conditions might give calculation values deviating substantially from the corresponding real values. Thus, incorrect fault indications might be generated based on test samples recorded during the last mentioned operating conditions.
The functioning of an SCR catalyst may also be monitored by means of a NOx sensor arranged to measure the NOx content in the exhaust gases downstream of the catalyst. This NOx sensor may be supplemented by another NOx sensor arranged to measure the NOx content in the exhaust gases upstream of the catalyst in order to provide a comparison value. However, a NOx sensor is an expensive component and one option for dispensing with a NOx sensor upstream of the catalyst is to use a calculation model for calculating the amount of NOx produced by the vehicle engine. Furthermore, another calculation model may be used for calculating the expected conversion of NOx in the catalyst so as to obtain calculated values of the NOx content in the exhaust gases downstream of the catalyst. These calculated values may then be compared with the measuring values from the NOx sensor downstream of the catalyst in order to detect possible disorders of the catalyst and the associated equipment. The use of calculation models for calculating the magnitude of the NOx content in the exhaust gases may however give calculation values deviating substantially from the corresponding real values, which in its turn may result in the generation of incorrect fault indications.